In electric power transmission and distribution networks, fault current conditions may occur. A fault current condition is an abrupt surge in the current flowing through the network caused by faults or short circuits in the network. Causes of the faults may include lightning striking the network, and downing and grounding of the transmission power lines due to severe weather or falling trees. When faults occur, a large load appears instantaneously. The network, in response, delivers a large amount of current (i.e. overcurrent) to this load or, in this case, the faults. This surge or fault current condition is undesirable as the condition may damage the network or equipments connected to the network. In particular, the network and the equipment connected thereto may burn or, in some cases, explode.
One of the systems used to protect power equipment from damage caused by fault currents is a circuit breaker. When a fault current is detected, the circuit breaker mechanically opens the circuit and disrupts overcurrent from flowing. A Circuit Breaker typically take 3 to 6 power cycles (up to 0.1 seconds) to be triggered. During this time, damage can be done to the transmission line, transformers, and switchgear.
Another system to limit the fault current is a superconducting fault current limiter (“SCFCL”) system. Generally, a SCFCL system comprises a superconducting circuit that exhibits almost zero resistivity below a critical temperature level TC, a critical magnetic field level HC, and a critical current level IC. If at least one of the conditions is raised above the critical level, the circuit becomes quenched and exhibits resistivity.
During normal operation, the superconducting circuit of the SCFCL system is maintained below TC, HC, and IC. During a fault, one or more the conditions is raised above the critical level TC, HC, and IC. Instantaneously, the superconducting circuit in the SCFCL system is quenched and its resistance surges, thereby limiting transmission of the fault current. Following some time delay and after the short circuit fault is cleared, To, Ho and Io are returned to normal values and current is transmitted through the network and the SCFCL system.
The SCFCL system may comprise an enclosure electrically decoupled from ground, such that the enclosure is electrically isolated from ground potential. In other embodiments, the enclosure may be grounded. The SCFCL system may also have first and second terminals, electrically connected to one or more current carrying lines; with a first superconducting circuit contained in the enclosure, where the first superconducting circuit electrically connected to the first and second terminals.
Referring to FIG. 1, there is shown an exemplary superconducting fault current limiting (SCFCL) system 100 according to the prior art. The SCFCL system 100 may comprise one or more modules 110. For the purposes of clarity and simplicity, the description of SCFCL system 100 will be limited to one single phase module 110.
The module 110 of SCFCL system 100 may comprise an enclosure or tank 112 defining a chamber therein. In one embodiment, the enclosure or tank 112 may be thermally and/or electrically insulating tank 112 such as those made with fiberglass or other dielectric material. In another embodiment, the tank 112 is a metallic tank 112 comprising an inner and outer layers 112a and 112b, and a thermally and/or electrically insulating medium interposed therebetween. In this case, the tank 112 is not connected to earth ground, and is referred to as a floating tank configuration.
Within the tank 112, there may be one or more fault current limiting units 120 which, for the purpose of clarity and simplicity, are shown as a block. The module 110 may also comprise one or more electrical bushings 116. The distal end of the bushings 116 may be coupled to a respective current line 142 via terminals 144 and 146 to couple the SCFCL module 110 to the transmission network (not shown). The current lines 142 may be transmission lines used to transmit power from one location to another (e.g. current source to current end users), or power or current distribution lines. The inner conductive material in the bushings 116 may connect the terminal 144 and 146 of the bushing 116 to the fault current limiting unit 120. Meanwhile, the outer insulator is used to insulate the enclosure or tank 112 from the inner conductive material, thereby allowing the tank 112 and the terminals 144 and 146 to be at different electrical potentials. If desired, SCFCL module 110 may comprise optional internal shunt reactor 118 or an external shunt reactor 148, or both, to connect the conductive material contained in the electrical bushings 116.
Several insulated supports may be used to insulate various voltages from one another. For example, insulated supports 132 within the tank 112 are used to isolate the voltage of the module 120 from the tank 112. Supports 134 are used to isolate the platform 160 and the components resting thereon from ground.
The temperature of one or more fault current limiting units 120 may be maintained at a desired temperature range by coolant 114 in the tank 112. In one embodiment, it may be desirable to maintain the fault current limiting units 120 at a low temperature, for example, ˜77° K. To maintain at such a low temperature range, liquid nitrogen or another cryogenic gas may be used as the coolant 114.
This coolant 114 may be cooled using an electrical cooling system. This cooling system may include a cryogenic compressor 117 and a transformer 115. This cooling system may require 15-20 kW of energy to provide 1 kW of cooling at liquid nitrogen temperatures. Thus the transformer 115 may need to supply large amounts of power. Additionally, the transformer 115, which is typically at ground potential, outputs voltages in excess of 100 kV.
In addition, in the case of a floating tank configuration, it is necessary to supply that electrical power at a voltage at or above that of the tank 112. In transmission systems, the tank 112 may be biased at 110 kV, 220 kV, 345 kV or 500 kV. Designing the transformers 115 needed to output this amount of power at these voltages is a significant challenge. In addition, to electrically isolate the floating tank, it is often necessary to place the tank 112 on a platform 160, which is elevated from the ground using insulated supports 134. This platform 160 may be 6-10 feet or more above the ground.
Thus, an issue associated with this configuration is the need to provide electrical power at the potential of the current carrying lines, which may be as high as between 10 kV and 350 kV. As described above, this electrical power may be used to power the cooling system required to keep the SCFCL system at the desired temperature. The typical solution to providing power across such large voltage potentials is the use of a transformer, which may be very large, such as hundreds of kV, and very expensive. Furthermore, such transformers are vulnerable to lightning strikes and other failure mechanisms.
Therefore, an improved system for providing electrical power to SCFCL systems is needed.